Efforts to provide physiological and biophysical knowledge applicable to evaluation and control of occupational exposures to heat are being continued along three lines: (a) A rational basis is being sought for arranging work-rest cycles for jobs which involve intermittent exposures to very hot environments such as are required in numerous industrial operations. (b) A prototype single-instrument heat stress meter has been designed and constructed Its performance as a predictor of heat strain on workers will be evaluated. (c) An attempt will be made to find reasons for present limitations in ability to predict tolerable limits for continuous work in hot environments. Present methods for equating heat load resulting from work and environment with calculated capacity for achieving evaporative cooling through sweating do not consistently achieve this purpose.